When starting internal combustion engines, which are based on the principle of the gasoline engine, the metering of the fuel quantity to be injected plays a decisive role in the ignitability of the air/fuel mixture as well as in the exhaust gas composition. The required fuel quantity depends greatly on the fuel characteristics, especially on the vaporization characteristics of the fuel.
An air/fuel ratio, which is too lean, can, for example, be detected via the evaluation of the engine rotational speed using a start quantity adaptation. Using an afterstart adaptation, the uneven running of the internal combustion engine is evaluated. Immediately following said adaptation, a poor vaporization behavior of the fuel can be compensated for with a mixture enrichment.
Internal combustion engines based on the principle of the gasoline engine are generally operated with fuel from hydrocarbons, which are derived from fossil fuels on the basis of refined crude oil. Alcohol produced from renewable resources (plants), for example ethanol or methanol, is increasingly being added in various mixing ratios to this fuel. In the USA and Europe a mixture of 75-85% ethanol and 15-25% gasoline is often distributed under the trade name E85. The internal combustion engines are designed in such a way that they can be operated with pure gasoline as well as with blends up to E85. This is denoted as a “flex-fuel operation”.
As a result of the arbitrary mixture of gas and alcohol, a broad spectrum of variations of the fuel characteristics arises, especially of the fuel stoichiometry and the vaporization behavior.
From the German patent DE 4117440 C2, a method is known for the adaptive adjustment of a fuel/air mixture to take into account the fuel characteristics in the operation of an internal combustion engine, which has a lambda controller, which emits a control factor RF, and which has an adaptation integrator, which emits and adaptation factor AF with variable adaptation speed. Said adaptation factor AF with the control factor RF influences the adjustment of the fuel/air mixture. In so doing, provision is made for a test to determine whether the lambda closed-loop control deviation amplitude exceeds a first threshold value. If this is the case, the adaptation speed is set at an increased value up until a specified condition is met. Thereafter it is shifted back to a lower adaptation speed.
The method makes the trouble-free operation of internal combustion engines possible, which can be operated with variable fuels. Thus, the duration of injection must, for example, be extended by more than 20%, when a change is made from a fuel of pure gasoline to a fuel containing 85% ethanol and 15% gasoline. In so doing, the same lambda values can be maintained in the exhaust gas. According to the method described in the text of the German patent DE 4117440 C2, a corresponding adaptation intervention is additionally performed. Because in comparison to the compensation for the influences of wear and manufacture, a very significant correction of the durations of injection and thereby of the adaptation intervention must be performed when a refueling of the tank takes place, the adaptation speed is significantly increased in the proposed method when a refueling has been detected.
On the basis of the adjusted adaptation value, the fuel mixture ratio can be determined. Despite the increased adaptation time, the method requires a sufficiently long settling time. If a significant change in the fuel mixture ratio is caused by the filling of the tank (fueling), this can lead to starting difficulties and to misfires, which in turn lead to increased exhaust gas emissions.
Beside a change of the fuel mixing ratio of an internal combustion engine operated in the flex-fuel mode, a variation in the fuel quality can also, for example, lead to difficulties in starting in the pure gasoline operating mode. In this instance, the fuel quality especially has an effect on the vaporization characteristics of the fuel. When the alcohol percentage in the fuel is not known, it is difficult to distinguish between the two aforementioned causes. An enrichment of the air/fuel mixture as a reaction to the poor starting performance of the internal combustion engine, as it is known from the pure gasoline operating mode, is for this reason not always appropriate when applied to the flex-fuel mode.
It is the task of the invention to provide a method, which allows for a reliable starting of the internal combustion engine when the fuel mixing ratio is not known or when the fuel quality is not known.